Earlier this year, a bunch of Poles ran an IndieGoGo campaign for a new larpwriting web app called "Spindle". Its plot-based rather than relationship-based, using a theory of larp "quantum" (which probably sounds less stupid / desperate in Polish). There's a presentation on it here.
One down side is that its pay-to-play for anything over 20 players, but its currently in open Beta, which means you don't have to pay to use it at all. You can sign up for it here:
Note that if you use this to write a large larp in the Beta phase, you'll need to pay them later if you ever want to edit it again. But it will be accessible and you will apparently be able to keep producing handouts for it even from a free account.